The present invention relates generally to protection and fault detection schemes and more particularly to rapid response protection and detection circuits for semiconductor power drivers.
Power semiconductors such as transistors are now commonly used as switches or drivers to supply electrical power to other operational devices such as d.c. contactors, lamps and relays. A very prevelant problem, however, exists with respect to the protection of the semiconductor device itself from overload currents which would be of a magnitude sufficient to destroy the device and to providing a reliable indication of a fault condition at a reasonable cost.
Fuses are one commonly used form of circuit protection and are often employed in semiconductor (including transistor) drive circuits such as discussed above. Fuses, however, are relatively slow reacting devices (e.g., in the 1 to 10 millisecond time range) while semiconductor devices can be destroyed by overload conditions in as little as one-tenth the reaction time of the fuse. Thus, while a fuse may serve to protect the circuit wiring, etc., its use as a sole protective device will usually permit the destruction of the semiconductor driver.
Another form of semiconductor protection sometimes used is that of sensing the load current carried by the semiconductor device and shutting the device off in the event of an overcurrent. While this form of protection can usually provide sufficiently rapid protection, accurate d.c. current sensing is a very difficult and often expensive function and, in situations such as lamp loads where there is a large inrush current, may also give misleading results.